Method and apparatus for measuring electric motor efficiency and loading

Abstract

A method for determining the efficiency and loading of an electric motor comprises affixing a strain gauge to the drive shaft, calibrating the strain gauge, measuring the input power of the motor, measuring the rotational speed of the drive shaft, transmitting an RF signal from the strain gauge through a rotary antenna to a stationary antenna and calculating the torque in the drive shaft under load. An apparatus for carrying out said method includes a device for determining the input power of the motor, a device for determining the rotational speed of the drive shaft, a strain gauge affixed to the draft shaft coupled to an RF transmitter transmitting a signal through a rotary antenna affixed to the drive shaft, and transmitting device coupled to the stationary antenna for transmitting an RF signal to the rotary antenna and receiving an RF signal from the rotary antenna proportional to the amount of strain experienced by the strain gauge. The output power of the motor can thus be determined under load conditions without decoupling the motor from the load, and related to the input power to determine the efficiency of the motor.

I claim: 
     
       1. A method for determining the loading of a motor having a drive shaft connected to a load, comprising: affixing a strain gauge to the motor drive shaft and calibrating the strain versus torque relationship using a known dead weight applied over a known length of drive shaft to obtain a measure of the strain versus torque relationship,   using a power transducer to measure the power input P in  of the motor under load conditions,   using a speed sensor to measure the rotational speed N of the drive shaft, and   transmitting an RF signal from the strain gauge through a rotary antenna to a stationary antenna to obtain a strain reading and dividing the strain reading by the strain versus torque relationship to obtain a measurement of the torque T in the drive shaft under load.   
     
     
       2. The method of claim 1 including the further steps of calculating the efficiency of the motor, and recording efficiency and loading data to provide a motor load profile. 
     
     
       3. The method of claim 2 wherein signals representing the power input, rotational speed of the drive shaft and torque on the drive shaft are electronically recorded and related by a data recorder. 
     
     
       4. The method of claim 3 wherein the signals are recorded in intervals of approximately fifteen minutes. 
     
     
       5. The method of claim 4 including measuring the temperature of stator windings in the motor to ensure that successive load determinations have been calculated under like conditions. 
     
     
       6. The method of claim 1 wherein the rotational speed of the drive shaft is determined using a photosensor. 
     
     
       7. An apparatus for determining the efficiency and/or loading of a motor having a drive shaft connected to a load, the loading being derived from a measurement of torque and the efficiency being derived from a measurement of torque multiplied by the rotational speed of the drive shaft and divided by the input power of the motor, comprising: means for determining the input power of the motor,   means for determining the rotational speed of the drive shaft,   a strain gauge for producing a measurement of relative torque affixed to the drive shaft and coupled to an RF transmitter transmitting an RF signal proportional to the amount of strain on the strain gauge through a rotary antenna affixed to the drive shaft, and   means coupled to a stationary antenna for receiving the RF signal from the rotary antenna, wherein the transmitter is located in a collar forming a rotary antenna detachably clamped to a the drive shaft, wherein the collar comprises a compression clamp spaced from the drive shaft by insulating spacer blocks, and the transmitter is contained within one of the spacer blocks and electrically coupled to the clamp.     
     
     
       8. The apparatus claimed in claim 7 wherein means for determining the rotational speed of the drive shaft comprises a photosensor. 
     
     
       9. The apparatus claimed in claim 7 including a data recorder for receiving and recording signals produced by the means for determining input power, the means for determining rotational speed of the drive shaft and a strain gauge affixed to the drive shaft for determining torque in the drive shaft. 
     
     
       10. The apparatus claimed in claim 7 including a thermocouple located near stator windings of the motor outputting a signal reflecting the temperature of the stator windings. 
     
     
       11. A method for determining the torque of a motor having a drive shaft connected to a load, comprising: affixing a strain gauge to the motor drive shaft and calibrating the strain versus torque relationship using a known dead weight applied over a known length of drive shaft to obtain a measure of the strain versus torque relationship, and   transmitting an RF signal from the strain gauge through a rotary antenna to a stationary antenna to obtain a measurement of strain and dividing the strain measurement by the strain versus torque relationship to obtain a measurement of the torque T in the drive shaft under load.   
     
     
       12. The method of claim 11 including the further step of recording the torque measurement at intervals to provide a motor load profile. 
     
     
       13. The method of claim 12 wherein signals representing the torque on the drive shaft are electronically recorded by a data recorder. 
     
     
       14. The method of claim 12 wherein the signals are recorded in intervals of approximately fifteen minutes. 
     
     
       15. The method of claim 14 including measuring the temperature of stator windings in the motor to ensure that successive torque measurements have been determined under like conditions. 
     
     
       16. The method of claim 11 wherein rotational speed of the drive shaft is determined using a photosensor.